In my experience of 2015 RPM, when a kite is charged in a jump it sit back more of 2014 at the zenit. That create more lift.
The same in a load &pop : in a loading it sit back in the window, it resist better and it's ready to shoot forward in a pop.

After 5 years ofI enjoying RPM I had 2015 13 and 11 Fuel for a couple of hours, with and without bridles. In my opinon they are enough different, also both bridled, they are not comparable: Rpm works on back lines more of a bridled Fuel.